The present invention relates to a novel vinyl•cis-polybutadiene rubber produced by concurrently allowing 1,2-polybutadiene of a high melting point of 170° C. or more and polyisoprene or polybutadiene of a low melting point to exist and be dispersed in the matrix of cis-polybutadiene rubber. Further, the invention relates to a butadiene rubber composition using the vinyl•cis-polybutadiene rubber.
In the molecular chain of polybutadiene, a binding portion generated by 1,4 polymerization (1,4 structure) and a binding portion generated by 1,2 polymerization (1,2 structure) concurrently exist as so-called microstructure. The 1,4 structure is divided in two types of structures, namely cis structure and trans structure. Alternatively, the 1,2 structure takes a structure with vinyl group as a side chain.
A method for producing vinyl•cis-polybutadiene rubber composition in the related art has been carried out in inert organic solvents such as aromatic hydrocarbons such as benzene, toluene and xylene and halogenated hydrocarbons thereof for example chlorobenzene. When solvents such as aromatic hydrocarbons and halogenated hydrocarbons are used, however, the resulting polymerization solution has such a high viscosity that the agitation, heat transmission and transfer thereof are troublesome, which requires excessive energy for the recovery of such solvent. Additionally, solvents such as aromatic hydrocarbons and halogenated hydrocarbons are very hazardous for environment, due to the toxicity and carcinogenesis.
As the production method, a method including a step of producing cis-polybutadiene rubber by cis-1,4 polymerization of 1,3-butadiene using a catalyst obtained from water, a soluble cobalt compound and an organic aluminum chloride represented by the general formula AlRnX3-n (provided that R is an alkyl group with one to 6 carbon atoms, phenyl group or cycloalkyl group; X is a halogen element; and n is a numerical figure of 1.5 to 2) in the inert organic solvent, and a step of syndiotactic 1,2 polymerization (abbreviated as “1,2 polymerization” hereinafter) of 1,3-butadiene in the presence of a syndiotactic 1,2 polymerization catalyst obtained from a soluble cobalt compound, an organic aluminum compound represented by the general formula AlR3 (provided that R is an alkyl group with one to 6 carbon atoms, phenyl group or cycloalkyl group) and carbon disulfide, with addition or no addition of 1,3-butadiene and/or the solvent to the resulting polymerization system is known (see for example JP-B-49-17666 (patent reference 1) and JP-B-49-17667 (patent reference 2)).
Additionally, for example, JP-B-62-171 (patent reference 3), JP-B-63-36324 (patent reference 4), JP-B-2-37927 (patent reference 5), JP-B-2-38081 (patent reference 6), and JP-B-3-63566 (patent reference 7) describe methods including a step of producing vinyl-cis-polybutadiene rubber composition by cis-1,4 polymerization of 1,3-butadiene in the presence or absence of carbon disulfide, and methods including a step of separating and recovering 1,3-butadiene and carbon disulfide to recycle 1,3-butadiene substantially never containing carbon disulfide and the inert organic solvent. Further, JP-B-4-48815 (patent reference 8) describes a rubber composition with a small die swell ratio of compounded material, of which the vulcanized product has an excellent tensile strength and a great flex-crack-growth resistance preferable as the sidewall of tire.
Additionally, JP-A-2000-44633 (patent reference 9) describes a method for producing vinyl•cis-polybutadiene rubber in an inert organic solvent containing C4 distillates such as n-butane, cis-2-butene, trans-2-butene, and butene-1 as the main components. JP-A-2000-44633 describes that 1,2-polybutadiene contained in the rubber composition according to the method is a crystal in short fiber, where 98% or more of the fiber length is less than 0.6 μm in the distribution of the crystal in short fiber along major axis; and 70% or more thereof is less than 0.2 μm and that the resulting rubber composition has improved moldability of cis-1,4-polybutadiene, tensile stress, tensile strength and flex-crack-growth resistance.
However, a rubber composition with improved various properties has been demanded for some use.